1. Field of Invention
Iso-alpha acids (isohumulones) -- isomerization of alpha-acids (humulones) thereto.
2. Prior Art
Hops have been used for centuries for the purpose of influencing microbial flora during fermentation of wort into beer, and for contributing a clean bitter flavor and rich aroma to the beer. As the chemistry of the hop constituents began to be elucidated, it became clear that the most important bittering compounds derived from hops were a group of isomers and analogous compounds, generally known as isohumulones or iso-alpha acids. The iso-alpha acids (isohumulones) are formed from alpha acids (humulones) during boiling of the wort in the traditional brewing process, and have been made in the laboratory and in commercial practice by a number of somewhat similar procedures.
The basic equation for the isomerization is depicted in Table 1, and it is clear that this equation applies to the common forms of alpha acids (cohumulone, adhumulone, and humulone) as well as to reduced form thereof, and other forms in which the ethylenic linkage in the side chain or chains may shift or become hydrated, as well as forms in which the alkyl and/or alkenyl side chains have different lengths or types of branching. In this specification, the terms "alpha acids" and "iso-alpha acids" are used to describe these types of compounds, as the critical portion of their structure as well as their behavior in terms of the basic equation of Table 1 is identical.
While work on elucidation of the structure of these hop bittering substances went forward, progress was also made in the field of hop extraction. Today, hop extracts are used interchangeably with hops by many brewers, and have the distinct advantage of greater uniformity, stability, and ease of storage. Hops may be extracted by any non-reactive solvent, such as methanol, ethanol, acetone, hexane, or lower-boiling chlorinated solvents.
A disadvantage in the use of whole hop extract, shared by the use of whole hops, is the low rate of conversion of alpha acids to iso-alpha acids during boiling of the wort, and the consequent overall low utilization of the hops in the beer. The efficiency with which hops are used when whole or in the form of whole hop extract is generally accepted to be about 30%.
A process for increasing efficiency of whole hops utilization is described in U.S. Pat. No. 3,451,821. This process involves isomerization of the whole hop extract to what is known as preisomerized hop extract, separating the iso-alpha acids therefrom, adding the non-isomerizable portion of the hop extract to the wort during boil (to secure the non iso-alpha acid bittering substances present, as well as to give the wort biological stability), and adding post-fermentation the iso-alpha acids and delicate hop volatile substances normally lost in the course of steam distillation during wort boil. In this way, utilization of the hops can exceed 75%, and at the same time greatly improved control is achieved by the brewer.
One of the drawbacks of known processes for producing a preisomerized hop extract is its cost, as the isomerization adds an expensive step to a relatively simple extraction process. For example, British Patent Specification 1,158,697 published 16 July 1969, shows catalytic isomerization of a 10% solution of hop extract in ethanol, when mixed with about four-tenths part of water, and at an elevated temperature. Such procedure requires the handling of about two pounds of ethanol and eight-tenths pound of water for each pound of hops extracted, and must therefore be carried out in extremely large volume isomerization reactors, e.g., approximately 5-10,000 gallon reactors. It also presents the formidable difficulty of removing the water and alcohol from the hop extract without damaging or losing the delicate aromatic compounds, following isomerization. The present invention eliminates the need for ethanol, or other water miscible co-solvent (coupling agent, as referred to by the British patentee), and permits the same solvent as used in the extraction of the hops, which is a water-immiscible organic solvent, to be used as the isomerizaton medium. Furthermore, according to the present invention, the hop extract may be isomerized in a relatively concentrated form, conveniently up to about 40% concentration in the extracting solvent, thus requiring only one-half to three-fourths pound of solvent per pound of hops for isomerization -- the advantage of an about five-fold reduction in volume of equipment required, as well as eliminating the necessity of introducing the previously-necessary water-miscible co-solvent for purposes of isomerization.
Another procedure for isomerizing the alpha acids to the iso-alpha acids is to separate the alpha acids from the hop extract by partitioning into water at selected pH ranges, as is also known to the art, and then to isomerize the alpha acids by heating at a pH of about 10. This eliminates the difficulties of removing water from the non-isomerizable resins which exist in the first procedure mentioned, and does not expose the other hop constituents to the conditions of isomerization, but it does require the additional step of separating the alpha acids, which is relatively difficult because the beta-acids (lupulones) are about as acidic as the alpha acids. This procedure also has the disadvantage of requiring large volumes of water, since the alpha acids are soluble in water only at a level of 1%-2% at the pH which must be used; thus about 4 to 5 pounds of water are required in this isomerization process for each pound of hops extracted. The present invention reduces the required volume capacity of the equipment employed by a factor of up to 10, as well as providing the other advantages to be mentioned in the following.
A further important shortcoming of known processes for desired isomerization of alpha acids to iso-alpha acids is the difficulty of controlling them so that no adverse side-reactions take place, involving either the alpha acids or iso-alpha acids, or other hop constituents. A discussion of known methods of isomerizing alpha acids to iso-alpha acids, and their shortcomings on a practical scale, will contribute to an understanding of the present invention.
As mentioned previously, the conventional method of isomerizing hops or hop extracts is to add them in brewing during boiling of the wort. In some cases, calcium sulfate is added and it is now known that this tends to increase the rate of isomerization if the wort is deficient in hardness. The same reaction occurs when pure alpha acids are boiled in hard water, but the yield is never satisfactory.
One of the earliest methods of isomerization was that reported by Carson, J. Am. Chem. Soc. 74, 4615 (1952), in which he refluxed alpha acids in methanol at a pH of 10. He formed both the typical bitter iso-alpha acids and a non-bitter oil, which has later been explained as being due to spiro compounds formed because of the presence of methanol [Maes et al., J. Inst. Brew. 76, 250 (1970)]. The behavior of ethanol is similar to methanol in this respect.
Another and more satisfactory method for effecting the desired isomerization is to heat an aqueous solution of alpha acids, or mixture of alpha acids and other hop resins, at a pH of 9.5 to 11.0 until the isomerization is complete. The difficulty of separating the alpha acids from the beta acids (in such type procedure) is already referred to above, as applied to commercial practice, and the other disadvantage is the difficulty of stopping the reaction before iso-alpha acids are hydrolyzed to undesired humulinic acid, which is non-bitter.
An improved method for the desired isomerization is reported in British Patent Specification 1,158,697. In this method, isomerization of the alpha acids is accelerated as a result of the presence of a polyvalent metal ion. Although this patent specification shows how to control the formation of humulinic acid by using a reduced reaction time and lower pH than employed in the alkaline isomerization, it does subject the hop constituents to an elevated temperature and to a water-miscible co-solvent, ethanol, which exposes the iso-alpha acids to conversion to Carson's non-bitter material. Although under the conditions described in the patent this may not form, it represents a significant hazard under commercial conditions in which heating and cooling times are and must be much longer. In addition, unless the ethanol can in some way be completely removed from the finished isomerized hop extract, it will form esters with acids present in the extract, and these affect the flavor of the beer as they are not uniformly metabolized by the yeast.
Thus it is seen that, in presently-known forms, all prior-art processes for converting a whole hop extract into a preisomerized hop extract, or for converting alpha acids into iso-alpha acids, have the following shortcomings:
1. They produce undesirable by-products, either tasteless or off-taste, unless very closely controlled.
2. They require large volume equipment and large volumes of solvent per unit of hops processed.
3. They require steps for solvent removal (and water removal) which are not required in producing whole hop extract.
4. They require elevated temperatures which expose the hop constituents, especially the alpha, iso-alpha, and beta acids, to oxidative reactions and hydrolysis to the non-bitter humulinic acid.